The present invention relates to a powder for use in a lower layer of a coating type magnetic recording medium having multilayer structure.
In a so-called coating type magnetic recording medium comprising a magnetic layer formed on a support by coating the support with a coating film of magnetic particles dispersed in a binder resin, it is desired to make the magnetic layer thinner to obtain high output characteristics while minimizing noise. To achieve this requirement, there is proposed a coating type magnetic recording medium having a multilayer structure, in which a non-magnetic layer (which is referred to herein as a xe2x80x9clower layerxe2x80x9d) is provided as a coating film of non-magnetic particles dispersed in a binder resin between the support and the magnetic layer.
As the non-magnetic powder for forming the lower layer, there has been proposed the use of spherical particles of titanium oxide or particles of iron oxide. For instance, in a magnetic recording medium having such a multilayer structure as disclosed in JP-A-Hei6-215360 (the term xe2x80x9cJP-Axe2x80x9d as referred herein signifies xe2x80x9can unexamined published Japanese patent applicationxe2x80x9d), there are disclosed the characteristic values for cases using acicular hematite (xcex1-Fe2O3), spherical hematite (xcex1-Fe2O3), Co-xcex3-Fe2O3, acicular TiO2, or spherical TiO2. Similarly, JP-A-Hei6-139553, JP-A-Hei7-282443, JP-A-Hei7-326037, and JP-A-Hei7-334835 disclose the characteristic values for cases using acicular xcex1-Fe2O3 for the non-magnetic powder for forming the lower layer. Further similarly, JP-A-Hei7-78331, JP-A-Hei7-105530, JP-A-Hei7-182649, and U.S. Pat. No. 5,496,622 disclose the characteristic values for cases using Co-xcex3-Fe2O3, or still further, for cases using xcex1-Fe2O3. In the aforementioned publications, various names of substances other than the above iron oxide powders which are provided specifically with the characteristic values, are exemplified as other non-magnetic powders usable for the lower layer. The name of iron oxyhydroxide is also included in those of the substances. On the other hand, in JP-A-Hei4-167225 is disclosed a special type of multilayer structure, which is different from the others in that the lower non-magnetic layer is provided thinner than the upper magnetic layer. In the disclosure, evaluation is made on the resistance against flaws, stiffness, and head contact properties for a case using particles of iron oxyhydroxide as the powder for forming the lower non-magnetic layer. Furthermore, U.S. Pat. No. 5,637,390 discloses an example using xcex1-FeOOH powder coated with Si and Al as the lower non-magnetic powder, and the surface roughness of the tape using this powder is evaluated therein. The disclosure in JP-A-Hei6-60362 teaches that such iron oxyhydroxide is unfeasible as a powder for use in a lower layer because of its poor dispersibility in binder, and that the use of acicular xcex1-Fe2O3 is recommended.
In the coating type magnetic recording medium having a multilayer structure known thus far, it has been commonly believed that the use of an iron oxyhydroxide powder cannot sufficiently exhibit the advantage of employing the multilayer structure due to the poor dispersibility of the powder in the binder. Accordingly, it is not fully understood what type of iron oxyhydroxide enables the desired function when used as a powder for the lower layer of a magnetic recording medium. In fact, U.S. Pat. No. 5,637,390 discloses the properties of the magnetic recording medium in case of using the iron oxyhydroxide as the powder for the lower layer, however, it does not teach how the variation in the type or the shape of iron oxyhydroxide depending on the difference attributed to the production process of iron oxyhydroxide influences the characteristics of the powder used in the lower layer.
On the other hand, although iron oxyhydroxide is generally produced by oxidizing a suspension of Fe(OH)2, it is well known that a slight fluctuation in the condition of oxidization changes the generating phase as to change the characteristics and the morphology of the resulting product. Accordingly, iron oxyhydroxides known to present not always exhibit the properties suitable as the powder for use in the lower layer above.
Accordingly, an object of the present invention is, in case of employing the powder of an iron oxyhydroxide as the lower layer powder above, to clarify how the physicochemical properties as well as the figure characteristics of the powder influence the surface smoothness, strength, magnetic properties, weather resistance properties, etc., of the magnetic recording medium, and to contribute to the improvement of the magnetic recording medium having a multilayer structure.
According to an aspect of the present invention, there is provided a powder for use in the lower layer of a coating type magnetic recording medium, comprising acicular particles substantially free from branching and having a mean major axis length in a range of from 0.01 to 0.5 xcexcm and a mean minor axis length in a range of from 0.01 to 0.05 xcexcm, provided that the acicular particles are flat acicular particles which, when cut in a direction perpendicular to the major axis, uniformly exhibit a cross section having a larger width to smaller width ratio of 1 or higher, preferably 1.5 or higher, and that the quantity of discharged H2O at 100xc2x0 C. is in a range of 2.0% by weight or less.
The flat acicular particles according to the present invention is made of iron oxyhydroxide preferably containing from 0.1 to 30% by weight of either or both of Al and Si, having a specific surface area in a range of from 10 to 300 m2/g and a tap density of 0.4 g/cm3 or higher, and having a decomposition temperature in air of 210xc2x0 C. or higher.
In order to produce an iron oxyhydroxide powder consisting of flat acicular particles substantially free from branching according to the present invention, there is employed a method which comprises adding an alkali hydroxide to an aqueous solution of a ferric salt at an equivalent value of 1.0 to 3.5 with respect to Fe3+ ion to form precipitates (neutralized precipitates) at a temperature of 5xc2x0 C. or higher; allowing iron oxyhydroxides to precipitate by holding the resulting suspension at a temperature higher than the former temperature; and separating the resulting precipitated iron oxyhydroxide from the suspension. In this method, flat acicular iron oxyhydroxide solid solutions containing Al (occluded Al) can be obtained by providing Al dissolved in the suspension from which the iron oxyhydroxide is precipitated therefrom, or in the solution prior to the precipitation. Furthermore, flat acicular iron oxyhydroxide covered with an Al compound can be obtained by adding a water-soluble aluminum salt or an aluminate to the suspension containing therein the precipitated iron oxyhydroxide.